The Carbon Cycle: Key Component of the Climate System, with Implications for Policy

"Large quantities of carbon are actively exchanged between the atmosphere and the other carbon storage pools, including the oceans, vegetation, and soils on the land surface. The exchange, or flux, of carbon among the atmosphere, oceans, and land surface is called the global carbon cycle. Comparatively, human activities contribute a relatively small amount of carbon, primarily to atmospheric carbon dioxide (CO2), to the entire global carbon cycle. Despite this relatively small contribution to the atmospheric carbon, the resulting perturbation to the carbon cycle is increasingly recognized as a main factor driving climate change over the past 50 years.

If humans add only a small amount of CO2 to the atmosphere each year, why is that contribution important to climate change? The answer is that some of the CO2 released to the atmosphere by human activities is not transferred to oceans, vegetation, and soils quickly enough to prevent CO2 concentrations in the atmosphere from increasing over time. Human activities are transferring fossil carbon—which took millions of years to accumulate—from a large, long-term carbon storage pool into the atmosphere over a relatively short time span, thereby affecting the global carbon cycle. As a result, the atmosphere contains approximately 46% more CO2 today than prior to the beginning of the Industrial Revolution. As the CO2 concentration of the atmosphere increases, the degree to which the atmosphere traps incoming radiation from the sun increases, which in turn contributes to further warming of the planet.

The increase in atmospheric CO2 concentration is mitigated to some extent by two relatively large (as compared to the atmosphere) reservoirs of carbon—the global oceans and the land surface. The global oceans and the land surface are considered net sinks for carbon because they currently take up more carbon than they release. Most of the total global carbon sink is referred to as the unmanaged, or background, carbon cycle. Very little carbon is removed from the atmosphere and stored or sequestered, by deliberate action (e.g., carbon capture and sequestration). If the oceans, vegetation, and soils did not act as sinks, then the concentration of CO2 in the atmosphere would increase even more rapidly than has been observed..."
Carbon Cycle